Systems to alert drivers to adjust speed to the posted limit or to levels lower than the posted limit, for example, to accommodate upcoming road hazards or construction, have generally been limited to the use of signs. Such signs must be physically placed in advance of the hazard or construction site by law enforcement personnel or highway road crews. If not properly placed or lighted, such signs can be missed by drivers, further adding to the hazard.
Since law enforcement or road personnel cannot continuously monitor all segments of a road, signs cannot be used to warn drivers of transient conditions such as smoke, fog, wind, intense precipitation and other weather conditions. Further, since signs must be physically placed into position by appropriate personnel, even when road personnel are aware of a hazardous condition, typically there is insufficient time for placement of such signs to alert drivers to take precautions.
Some highways employ radio broadcasts to alert drivers to upcoming hazards. These radio broadcast systems face problems similar to those encountered in sign based systems. The time required to prepare a broadcast and the inability to monitor all road segments simultaneously makes radio broadcasts warning systems of limited value. Further, these systems require the driver to tune the radio receiver to a designated AM or FM band frequency, not already assigned to another broadcaster. Many drivers do not use the radio or tune to the designated frequency channel. Those drivers who do tune to the designated frequency channel are distracted from observing traffic conditions, thereby increasing the risk of accidents.
In addition, in the conventional systems described above, it is not practical to use signs or radio broadcast messages to alert drivers of an approaching emergency vehicle. Thus, a more reliable system to alert drivers of such hazards and conditions without distracting the driver is desirable to improve safety.
Radars transmitting in the X, K, or Ka bands have been used by law enforcement agencies to enforce speed limits for some time. Many drivers employ radar detectors which alert the driver when the vehicle is being radiated by such a police radar. In response to the alert, the driver can verify his compliance with the speed limit or adjust vehicle speed to be within the posted limit. Thus, an infrastructure of transmitters and receivers already exists for sending signals to drivers. Some radar detectors, however, have the ability to distinguish police radars from other radar signals generated. Thus, in order to use the existing infrastructure, it is necessary to provide a signal which such "smart" radar detectors would not reject.